1. Field of the Invention
The present invention relates to a media transport mechanism in a printer and in particular to a high accuracy vacuum belt used in conjunction with pinch roller assemblies for precise media handling.
2. Description of the Prior Art
The prior art devices will be discussed in terms of printers, although the concepts may be equally applicable to other devices having vacuum belts in conjunction with pinch roller assemblies.
There has been a continuing need for precise media handling in a printer, and especially at the region of the media where the printing takes place. The prior art devices adopted one of many ways of transporting media through the printing area; however, each of them suffer from some inherent drawbacks which reduce the accuracy of the printing.
In one type of prior art device which uses a flat stationary platen, friction drive rollers alone are used. Typically, two sets of friction drive rollers are provided on two separate drive shafts. However, the use of friction drive rollers introduces printing inaccuracies due to some deficiencies. First, it is difficult to synchronize two drive shafts. A usual technique to overcome this problem is to slightly over-drive the exit rollers to ensure that the media is tensioned adequately. However, during the entrance and exit of the media, there are times when the media is held down by only one set of rollers. Inaccuracies may be introduced during the transition from one set of rollers to the other set. Second, inaccuracies may occur because the rollers may become deformed. Third, the leading and trailing edges of the media are not well controlled by the two sets of drive rollers.
In a second type of prior art device, a rotary platen is used to advance media through the printing area. However, since the printing surface is curved, the resulting print gap will vary, which will cause inconsistencies in printing.
In a third type of prior art device, a tractor feed device is used to transport continuous sheets of media across a printing area. However, the paper positioning accuracy is severely affected by the accuracy of the holes in the media. Furthermore, tractor feed is inappropriate for transporting individual cut sheets.